CommonJs

In Node.js, the module system is based on the CommonJS pattern. CommonJS is a module specification that defines a way to organize and share reusable JavaScript code. Node.js adopted the CommonJS module system to provide a structured approach to modularize code in Node.js applications.

Here's an explanation of how CommonJS modules work in Node.js:

1. Module Definition: Each Node.js file is considered a module. In a module file, you can define functions, objects, or variables that you want to make available to other modules. You do this by using the module.exports object.

// Example module file: greetings.js

function sayHello() {
  console.log('Hello!');
}

function sayGoodbye() {
  console.log('Goodbye!');
}

module.exports = {
  sayHello,
  sayGoodbye
};

In the example above, the greetings.js module exports two functions, sayHello and sayGoodbye, by assigning them to the module.exports object.

2. Module Import: To use the functionality provided by another module, you need to import it in your code. This is done using the require function and specifying the path to the module file.

// Example module usage: app.js

const greetings = require('./greetings');

greetings.sayHello(); // Outputs: Hello!
greetings.sayGoodbye(); // Outputs: Goodbye!

In the app.js file, the require function is used to import the greetings module. The greetings variable now holds the exported functions from the greetings.js module, and you can invoke them as needed.

3. Module.exports and exports: In Node.js, the module.exports object is the main object that gets exported from a module. However, there is a shorthand notation using the exports object that allows you to directly assign properties to module.exports.

// Example module file: math.js

exports.add = (a, b) => a + b;
exports.subtract = (a, b) => a - b;

In the math.js module file, the properties add and subtract are directly assigned to exports. This shorthand notation is equivalent to module.exports.add = ... and module.exports.subtract = ....

It's important to note that when using the shorthand exports notation, you cannot reassign the exports object itself. For example, the following will not work as expected:

// Incorrect usage of exports
exports = {
  // properties
};

To correctly export an object using exports, you need to assign properties directly to it, as shown in the previous example.

CommonJS modules provide a convenient way to organize and share code in Node.js applications. They allow you to encapsulate functionality into separate modules and import them when needed, promoting code reuse and maintainability.